Separator, battery with separator and method for producing a separator

ABSTRACT

A separator for battery and including a plate shaped structure of inorganic fibers is distinguished in that said separator is impregnated with a dispersion of colloidal inorganic nano particles that have been enriched in the crossing points of the fibers when solvent has been dried so as to form binding agent. The invention also concerns a battery including such a separator with high pressure on the active material and a method for producing such a separator.

FIELD OF THE INVENTION

The invention relates to a separator for a battery and a battery with atleast one such separator. It also relates to a method for producing sucha separator.

DESCRIPTION OF PRIOR ART

Batteries for starting engines, lighting, auxiliary power and the likeare electrochemical current sources having energy stored in electrodes.The electrodes form an electrochemical system consisting of at least onecathode (positive electrode connected to the positive pole of thebattery), at least one anode (negative electrode connected to thenegative pole of the battery) and electrolyte.

The most common storages system for the above purposes is the leadbattery and the nickel-cadmium battery. Several other systems are underdevelopment, i.a. Ni-MH, which replaces the NiCd battery. Said batterysystems have water based electrolyte but other systems require organicelectrolyte and there are even batteries with salt melts.

If, for example, through a powerful mechanical force, a cathode and ananode in the same battery would be pressed together, short-circuit couldoccur. A short-circuit can be so powerful that an explosion takes place.Therefore it is almost always the case that a separator wall must bepositioned between each cathode and anode. The separating wall (theseparator) must be electrically non-conductive, but porous to the extentthat a current can pass relatively unrestricted between the electrodes.

In certain constructions the separator may take up the entire distancebetween the electrodes, in particular if this distance is small. In somesystems, for example in the lead battery, the electrolyte participatesin the cell reactions and the amount of sulphuric acid must be adjustedto the capacity that is desired to extract from the battery. For thatreason the electrode distance may be made extra large and it can benecessary to manufacture a separator having ribs. These ribs would beprovided with such a height and construction that they support againstthe electrodes. Typical porosity of a separator intended for a batteryhaving water based electrolyte can be 50-75%.

The material in the separator varies depending on the composition of theelectrolyte. PVC is a common kind of material since it is chemicallystable in acid as well as in alkaline electrolyte. In more advancedbatteries, working at high temperatures, as an example boron nitridefelt may be used. In some cases electrodes are arranged such that theyare in a liquid form, for example the NaS battery, and when theelectrolyte is comprised of solid Al₂O₃ the separator has beeneliminated.

A particular material has come into use in lead batteries. I.e. microfine fibers of chemically resistant glass (C-glass) are formed to a mathaving the thickness 0.5 mm up to 2 mm and a porosity of about 95%. Sucha mat may contain a large amount of acid electrolyte but can easily bepressed together. Thus, for example, a pressure of only about 80 kPa isnecessary to press a glass wool separator (AGM-separator; AGM=AbsorptiveGlass Mat) from the thickness 1 mm to 0.5 mm.

An AGM-separator has two properties making it useful in lead batteries.The separator can, if it is put against the active material in thepositive electrode, prevent loose particles from the electrode fromfalling down to the bottom of the battery container where, in that case,short-circuits could relatively easily appear.

The second advantageous property is the ability to have the sulphuricacid distributed in the pores of the separator also if the separator isnot completely saturated with acid. This property makes it possible forthe oxygen which is formed at the positive electrode during charging topass through the separators and be reduced to water at the negativeelectrode so called oxygen gas recombination.

In particular in maintenance-free lead batteries these advantages areexploited, since it is possible to make the batteries closed with only avalve which opens if the gas pressure becomes too high. It is alsopossible to reach higher capacity per unit volume in that the so calledsludge space below the electrodes and the space above the electrodeshave been eliminated to a great extent.

The demands on the batteries and their application have resulted in manydifferent constructions. Concerning lead batteries there are two maintypes: batteries having pasted, flat positive electrodes and batterieshaving positive tubular electrodes. The latter encloses the positiveactive material (PAM) in a porous housing and PAM surrounds a currentsupplier of lead or a lead alloy. The tube surrounding PAM is in itselfa good support for the mass. A certain compression of PAM occurs in thatthe central current conductor corrodes and forms lead dioxide which hasa greater volume than lead. It is well known that these tubularelectrodes have a longer lifetime measured in numbers of cycles than thepasted flat electrodes. The reason for this is considered to be thepressure occurring through said expansion.

With repeated discharges of the electrodes in a lead battery, there isan expansion of the active material, whereby the electrodes will becomemore porous at the same time as the contact between the differentparticles becomes weaker. This expansion continues and goes on with anumber of discharges until the internal particulate contact has beenbroken.

This effect can be counteracted by providing a mechanical pressureagainst the electrode surfaces during charging as has been described fortubular electrodes. A certain expansion should, however, be allowed inorder that the active material be well utilized. Through the springaction of glass fibers in AGM separators this type of separator would bewell suited for this purpose. When, however, there is most often adesire to make the separator as thin as possible in order to have theinner resistance in the battery minimized, such a separator would bepressed together so much that the spring action effect would cease.Higher pressures than 80 kPa are not common. Thin (pressed together)separators, i.e. 0.5 mm and there around bring about risk ofshort-circuit over dendrites.

THE AIM AND THE MOST IMPORTANT FEATURES OF THE INVENTION

The aim of the present invention is to avoid the problems of the priorart and in particular to provide an improvement of the stability andmanageability of the separator material as well as the capacity andlifetime of the battery.

This aim is obtained in a separator and a battery according to the abovethrough the features of the characterizing portion of the respectiveindependent claims.

Separators according to the invention can be subjected to highmechanical pressure during assembly without the structure of theseparator collapsing.

Distinguishing for the invention is that the fibers in the separatorsare linked together in such a way that the separator can withstandmechanical load without losing the ability to essentially retain itsinitial thickness when the load is relieved. It is also the aim of theinvention that the fibers are not to move with respect to each other.Further, the invention concerns producing separators that can withstanda load of up to 300 kPa.

It is also distinguishing for the invention that linking together of thefibers is achieved through enriching, concentrating of nano particlesand, at drying the liquid phase (the solvent), subsequently bindingtogether thereof and of the fibers in the crossing points.

According to the invention, said nano particles are supplied to theseparators through addition of a dispersion of said particles in wateror another solvent, whereupon the separators are dried. Hereby is thusformed a stable and permanent bonding of the particles to each other inthe crossing points of the fibers which resists attack from theelectrolyte used in the battery in question.

The term colloidal nano particles is intended to mean particles havingsuch small size, in the nanometer area, that the particles aremaintained dispersed in the used liquid so that there will be formed astable colloid. The small size of the particles also contributes to theabove mentioned stable and permanent bonding really being formed.

By the surface of the particles in question having surface bound groupswith electrical charge, the particles will repel each other when theyare dispersed in the liquid phase (the solvent). At the removal of thesolvent the particles will come closer to each other and also to thefibers, and bonding bridges will be formed between the separateparticles which lead to the inventive stabilization.

An impregnating liquid with a binding agent of preferably SiO₂,comprising said colloidal nano particles, is supplied to the separatorin order to obtain impregnation of separators.

The invention is particularly applicable where a high mechanicalpressure is applied on electrodes and separators. The invention can beapplied in all batteries having separators but is described here inparticular for bipolar lead batteries for long lifetime cycling.

Besides said drying process, through heat treatment of the enrichedseparator at temperatures between about 300° C. and 700° C. results inconsiderably more rigidity of the material in the crossing points andthereby a more stable separator.

Especially, the inorganic fibers are made of glass, which is an economicand technically useful material. In particular the separator accordingto the invention can include AGM material. By further the dispersionincluding SiO₂ in a water solution, a material is obtained which bindsitself well onto the glass in the fibers as well as an economic andeasily manageable dispersion.

By the binding agent comprising between about 20 and 60% of the totalseparator weight, a good balance between strength and resilience isachieved, which is accentuated when the binding agent preferablycomprises between about 25 and 45% of the total separator weight.

The invention also concerns batteries, preferably bipolar leadbatteries, assembled with separators according to the above and alsopreferably under high pressure.

Further advantages are achieved through other aspects of the invention.

It is previously known from JP 2001283810 by impregnation of AGMseparators with a liquid containing dispersed particles to achieveseparators having particles positioned between the glass fibers in orderto obstruct penetration of dendrites. Hereby these separators may bemade thinner than what is customary. Hereby there is thus no enrichmentof the impregnating material in the crossing points of the fibers. It isnot stated that an increased flexibility is achieved or that theseparators per see could resist a higher pressure.

Another way of adapting the separator to a (small) electrode distance isdescribed by Brecht (U.S. Pat. No. 5,091,275 Feb. 25, 1992). A bindingagent of colloidal SiO₂ and a sulfate in water solution is supplied tothe separator. The separator is dried in a compressed state whereby SiO₂and the sulfate are united to a coagula. The separators are mounted incells between electrodes and upon adding the acid, the binding agent isdissolved. Thereby the separator swells and provides good contactbetween electrode and separator. It is, however, evident from thisdocument that this coagula is dissolved and is not binding together theglass fibers after supplying the acid.

An untreated AGM separator (AGM=Absorptive Glass Mat) intended hereinconsists to 100% of glass having high chemical strength. The fiberdiameter may be <1 um for 90% of the material. A separator consisting ofuntreated AGM is mechanically weak and has low tear resistance, inparticularly when it has been filled with sulphuric acid or water (wetstrength). A certain flexibility can be observed in the untreated AGMseparator: when it is loaded with weights and subsequently relived itwill retain its initial thickness after a while if the load has not beenso high that the glass fibers have been broken.

There is, however, a certain difference between loading of a dry and awet separator. The wet separator will subsequently be somewhat lesselastic and the pressure applied to electrodes and separators inproduction will be reduced.

The flexibility of the separators is, as mentioned above, essential forcapacity as well as lifetime of the batteries. A separator should beable to maintain a high, constant pressure onto the active materialsduring the lifetime of the battery but at the same time have aflexibility allowing the expansion of the active materials followingfrom discharge. When loading starts, thereafter, the separator shouldspring back in order to obtain a compression of the active materialsback to initial thickness. The present invention is directed againstachieving such flexibility.

Separators are often manufactured from plastics with a mix of poremaking substances. The glass fiber separators can be bound with organicsubstances. Organic compounds in contact with PbO₂ should, however, beavoided since they subsequently are oxidized to CO₂ which makes oxygengas recombination difficult in valve controlled batteries. According tothe invention, only inorganic compounds are used as separator materialand as impregnating agent (binding agent).

In order to achieve a mechanically strong separator having a certainflexibility and high porosity, according to a preferred embodiment ofthe invention, AGM separators are impregnated with a dispersion ofcolloidal SiO₂ in nano particle form.

Product having the trade name “BINDZIL” and “NYACOL” respectively, aremanufactured by EKA Chemicals with different concentrations of SiO₂ anddifferent particle sizes. Here has been chosen “BINDZIL 30/220” havingparticle diameter 15 nm but the invention is not for that reason limitedto either this quality definition or this manufacturer but concerns alsoother kinds of dispersed colloidal nano particles.

The glass fibers in the basic material for said separators is looselyput in coils and gives to the separator a certain flexibility whichoccurs when glass treads are straightened out under applied pressure.The SiO₂ particles which through the dispersion are supplied to theseparator will upon drying bind together the fibers in the crossingpoints and increased rigidity and resistant against mechanical pressureis obtained. Since not all fibers in the separator are bound in this waythere is, however, a certain part of the flexibility left.

“BINDZIL 30/220” is a 30% solution with respect to the contents of SiO₂and is before impregnation diluted to a solution including between 10and 50% of BINDZIL 30/220, (corresponding to 3.5-16.4% by weight SiO₂)preferably 20% of BINDZIL 30/220 (corresponding to 6.9% by weight SiO₂)or thereabout. The solution is supplied to the separator in an amount offor example about 10 ml/100 cm² at a separator thickness of about 0.85mm. The supplied volume may be modified and of course depends also onthe thickness of the separator. It has been proved advantageous to use asolution which has been obtained through dilution of between 15 and 35%BINDZIL 30/220, preferably about 25-30%, since this brings about apreferred balance between rigidity, flexibility and remaining porositywhich is suitable for most applications.

After drying at about 110° C. the separators, which before impregnationwere soft and flexible as a fabric, now have become rigid but withcertain flexibility. An additional rise of the temperature to at least300° C. and up to about 700° C. gives a very rigid separator. Separatorsthat have been impregnated this way can now be handled as plane sheetsat assembly of the batteries. In case of glass fibers, temperatures inparticular in the region about 500° C. are advantageous, since at highertemperatures the glass can be negatively affected.

The above defined percentages are related to BINDZIL 30/220. A morepractical measure is to define percent added binding agent i.e. amountdry SiO₂. In table 1, therefore, “% BINDZIL” has been noted also as“gram SiO₂/gram glass”. The porosity in AGM separators is high (about95-96%) and is effected very little by the added material. Here is alsoshown the relationship between amount SiO₂ and porosity. TABLE 1 BINDZILSurface weight Binding agent SiO₂/glass Porosity (%) *) (g/m²) (%) (g/g)(%) 0 132 0 0 95 10 168 21 0.27 94.6 20 204 35 0.54 93.2 50 312 58 1.3689*) Concerns % BINDZIL 30/220 in water solution for example 20% = 20 mlBINDZIL + 8 + ml aq.dest.

In the displayed examples and in general it has been discussed hereabout micro glass as separator material. At occurrence separators mayalso be manufactured based from other mineral fibers. These may be boundtogether in the same way with colloidal SiO₂ but also with colloidalparticles of Al₂O₃, Al(OH)₃, TiO₂ and moreover also most other metaloxides can be suitable binding agents and are therefore included in theinvention. As an example Al₂O₃ fibers are bound by colloidal SiO₂ andalso by Al(OH)₃ and TiO₂. A great number of other combinations of fibermaterials—impregnation agents/binding agents can be used and areincluded in the invention.

The solvent for the colloidal SiO₂ is water with pH about 9.0. It ispossible that also organic solvents could be used and the invention alsoincludes these.

Lead batteries may be arranged such that PAM is subjected to a certainmechanical pressure which resists an expansion of PAM. At the same timeas pressure is applied against PAM the same pressure occurs on thenegative active material (NAM). Since NAM, which in a charged state iscomprised of porous lead, is softer than PAM, NAM will be reduced inthickness if no measures are taken. In order to compensate for thisdrawback, according to the invention a pressure absorbing grid isincluded into the negative electrode.

Batteries with a pressure of up to 80 kPa on AGM separators placedbetween PAM and NAM are previously known. According to the invention itis possible to combine high mechanical application pressure on theelectrodes with an impregnated separator of AGM type and a pressureresisting device at the negative electrode. This device may be apressure molded grid or protrusions in the intermediate wall in bipolarbatteries. In common batteries this pressure at the negative electrodeis most often no problem, since NAM is supplied to the negative gridalong its outer contour.

DESCRIPTION OF EMBODIMENTS

The application of the invention will here be described in connectionwith a bipolar lead battery intended for discharging and charging withhigh current. This does, however, not limit the invention to thisembodiment since it is considered that the invention may be adaptedfirstly to every other construction of lead batteries but also to othertypes of batteries. The drawings show:

FIG. 1 diagrammatically a bipolar battery,

FIG. 2 in a diagram the compression of AGM separators with and withoutimpregnation at increasing and decreasing load,

FIG. 3 a grid which is intended for resisting pressure at the negativeelectrode,

FIG. 4 a semi-bipolar battery unit,

FIG. 5 the lifetime of a bipolar battery having separators according tothe invention,

FIG. 6 a an electron microscope photograph of glass fibers in anuntreated glass fiber mat, and

FIG. 6 b an electron microscope photograph of how SiO₂ binds togetherglass fibers a in glass fiber mat according to the invention.

What is said below about glass fibers formed into a separator forbatteries is also true for other inorganic compounds that can be formedinto fibers.

The invention concerns a reinforced separator for battery, batterieshaving said separators and a method of producing such separators. Suchbatteries can have a mechanical pressure on the electrodes of betweenabout 80 and 250 kPa and a pressure resisting device in the negativepart, preferably of plastic. The separators shall withstand saidpressure without the material breaking and shall have a certainflexibility.

A battery for high currents corresponding to discharge times of about0.5 to 1 minute for complete discharge should have a short electrodedistance in order for the inner resistance inside a lead battery to below. Further, the electrode and the other components of the batteryshould be constructed such that an even distribution of the current overthe electrode surfaces is obtained. A preferred embodiment of such abattery can be a bipolar construction as for example is known from U.S.Pat. No. 5,510,211. This battery is constructed for said charging anddischarging situation. It has been shown that a mechanical pressure ofat least 150 kPa but preferably 200 kPa gives a battery with a goodlifetime. The description of the invention will adjoin to said patent,but is for that reason not necessary bound to that construction.

With reference to FIG. 1, an electrode 1 for bipolar batteries includesan electron conducting wall 6 having PAM 5 and NAM 7 on each side ofthis wall. Each bipolar electrode 1, in particular in batteriesaccording to said U.S. Pat. No. 5,510,211 is fitted in a frame 2 whichis constructed such that it gives room for a separator 4. Five bipolarelectrodes and two monopolar end electrodes 2 together form a 12 Vbipolar battery. The walls 0.6 are comprised of porous chemical disks(for example 20×15 cm) the pores of which are filled with lead or a leadalloy in order to obtain electric conductivity.

The negative mass which comprises a mix of lead oxide, water, sulphuricacid and so called expander is applied in a wet state onto one side ofthe ceramic lead-filled disk which has a pressure relieving grid (seealso FIG. 3; 9 concerns spaces for receiving the active mass in thestructure 10) to a thickness of about 1 mm and not exceeding thethickness of the grid.

The positive mass may be comprised of a mix of water andpre-manufactured tetra basic lead sulphate (4PbO.PbSO₄) and is suppliedat the other side of the bipolar electrode and against the lead filledporous ceramic disk. After drying a forming process is carried outwhereupon the negative mass is transformed into porous Pb and thepositive mass into porous PbO₂ in a way that is well known to personskilled in the art.

Separators 4 somewhat larger than the electrode surfaces and having athickness of 0.85 mm are prepared with BINDZIL 30/220 as is describedaccording to an example below. Separators are dried at 110° C. overnight. At assembly, which is made with a separator between everyelectrode, the separators are compressed through the pressure to 0.7 mm.

After forming and rinsing, end electrodes are mounted having poles,bipolar electrodes and separators together into a pile and are pressedtogether with the aid of tension rods to pressure of 200 kPa.

Other pressures can be chosen wherein the separator is impregnated witha greater or smaller amount of BINDZIL in the impregnating liquid whichis illustrated in FIG. 2. This figure shows the compression as afunction of loading pressure. The load was increased stepwise with about25-50 kPa until the separator was entirely compressed. Thereafter theseparator was unloaded stepwise, whereby the thickness increased.

From the figure it is obvious that a non-impregnated separator iscompressed to 0.7 mm already with about 15 kPa, whereas with a 20%BINDZIL (=0.42 g SiO₂/gram glass) 100 kPa is reached and with 50%BINDZIL (1.05 g/g) about 180 kPa. In order to reach the pressure 250 kPawith non-impregnated separators it is required to have two separators,each having the thickness 0.85 mm, that are compressed to 0.7 mm.

In another preferred embodiment, see FIG. 4, the bipolar electrode isproduced in two halves. One half comprising the positive part of thebipolar electrode with active material applied on the lead-infiltratedceramic disk, and the other comprising the negative part with activematerial put on a leaded copperplate 10 with a grid for pressure relief.

The electrode halves are included in a frame each and put together toform a space for the separator. A separator 4 according to the inventionimpregnated with BINDZIL is placed between these electrodes. Theseparator has a thickness of for example 0.85 mm and is compressed to0.7 mm which requires a pressure of 200 kPa if the amount impregnationis 50% BINDZIL. These electrodes with their separator are sealed undercompression with heat, or in any other manner which is well known to theperson skilled in the art, into one unit of 2V. This unit and anoptional number of units manufactured in the same way are put togetherinto a pile and are driven against each other with tension rods so thatgood electric contact is obtained between all units.

By observation in an electronic microscope it can be clearly seen thatmost of the crossing points of the glass fibers have been locked bydried SiO₂, FIG. 6 b. This locking is surprisingly stable, probablydepending on that the basic material as well as the supplied suspensionhas the same basic composition. The chemical stability is also verygood: a piece of AGM was impregnated with 30% BINDZIL 30/220 solution(corresponding to 0.52 g/g) and was given a number of 900 folds in wetstate and was dried at 110° C. over night. The specimen was then kept insulphuric acid having the density 1.30 for 12 months. No change of shapeor ability to resist pressure could be observed after this time. As acomparison, in FIG. 6 a a corresponding glass fiber structure is shownin untreated state.

EXAMPLE 1

Two bipolar batteries of 4V with electrode surface of 16.6 cm² weremounted with on the one hand (A) two impregnated separators of AGM type,each of a thickness of 0.85 mm, on the other hand (B) a separator of AGMtype, thickness 0.85 mm impregnated with 27% BINDZIL. The separators ofboth cells were compressed to 0.7 mm (electrode distance), the firstbattery with 250 kPa and the later with 150 kPa. The batteries werecycled as follows: 10 s discharge with 5.4 A+25 s charge with 2.16 A+5 srest etc. for 20 hours, whereupon the batteries were fully chargedduring 4 hours. Thereafter the cycling continued. Every other weekdischarge was made with 0.3 A for determining capacity. The dischargingtime as a function of the number of cycles are shown in FIG. 5. From thefigure is clear the considerable difference in practical lifetime of abattery according to the invention in comparison with a moreconventional battery. In practice one treated separator is also superiorto two which are untreated.

EXAMPLE 2

A separator with 27% BINDZIL was manufactured by an un-impregnatedseparator of AGM type 20.5×13.5 cm×0.85 mm thick was put on a perforatedaluminum plate which was somewhat larger than the separator. A BINDZILsolution was prepared by 27 ml BINDZIL 30/220 was diluted into 100 ml.26 g of this solution was supplied to the separator from the centretowards the edges. Finally, the aluminum plate with the separator wasput inclining and an additional 1 gram of the solution was applied alongthe upper edge. The separator was covered with an aluminum plate of thesame kind as it was resting on. The separator was dried in an oven at110° C. over night.

1. Separator for battery including a plate shaped structure of inorganicfibers, characterized in that said separator is impregnated with adispersion of colloidal inorganic nano particles that have been enrichedin the crossing points of the fibers so as to form binding agent when asolvent of the dispersion has dried.
 2. Separator according to claim 1,characterized in that the separator has been heat treated at atemperature between 300 and 700° C. in order to obtain a considerablygreater rigidity.
 3. Separator according to claim 1 or 2, characterizedin that the inorganic fibers comprise material of any of the group:glass fiber, mineral fiber, metal fiber.
 4. Separator according to claim1, 2 or 3, characterized in that the binding agent includes any of thegroup: SiO₂, Al₂O₃, Al(OH)₃, TiO₂.
 5. Separator according to any of theprevious claims, characterized in that the binding agent comprisesbetween about 20% and 60% of the total separator weight.
 6. Separatoraccording to claim 5, characterized in that the binding agent comprisedbetween about 25 and 45% of the total separator weight.
 7. Battery withpositive and negative electrodes, separators and electrolyte,characterized in that it includes at least one separator according toany of the claims 1-6.
 8. Battery according to claim 7, characterized inthat it is mounted with a pressure at its electrodes of at least 100kPa, preferably 150-250 kPa.
 9. Battery according to claim 7 or 8,characterized in that binding agent has been supplied to the separatorin such an amount that it is compressible to about 80% of its thicknessat an outside applied pressure of between 80 and 250 kPa.
 10. Batteryaccording to claim 7, 8 or 9 in bipolar form, characterized in that apressure relieving grid is positioned in each negative electrode. 11.Battery according to any of the claims 7-10, characterized in that it iscomprised of a lead battery with sulphuric acid electrolyte.
 12. Methodof producing a separator for a battery, wherein a disk-shaped structurecomprising inorganic fibres is used, characterized in that saidseparator is impregnated with a dispersion of colloidal inorganic nanoparticles which are enriched in the crossing points of the fibres so asto form binding agent when a solvent of the dispersion is made to dry.13. Method according to claim 12, characterized in that drying ofsolvent is obtained through drying at raised temperature.
 14. Methodaccording to claim 12 or 13, characterized in that the separator afterdrying the solvent is heat treated at a temperature between 300 and 700°C. in order to obtain a considerably greater rigidity of the bond insaid crossing points.
 15. Method according to claim 12, 13 or 14,characterized in that the inorganic fibres including material from anyof the group: glass fibres, mineral fibres, metal fibres are used. 16.Method according to any of claims 12-15, characterized in that bindingagent from the group: SiO₂, Al₂O₃, Al(OH)₃, TiO₂ is used.
 17. Methodaccording to any of claims 12-16, characterized in that the bindingagent is brought to comprise between about 20 and 60% of the totalseparator weight.
 18. Method according to any of the claims 12-17,characterized in that the binding agent is brought to comprise betweenabout 25 and 45% of the total separator weight.